O'Donnell, a former television political analyst and tea party candidate, achieved an unlikely outcome despite being the target of a ferocious battery of assaults by Castle and the Republican Party targeting her personal finances, college education and ability to oversee public resources. Analysts portrayed the result as seriously endangering the GOP's effort to beat Democrat Chris Coons, the New Castle County executive, in November to capture Vice President Joe Biden's Senate seat.

Castle flatly warned voters in the days leading up to the primary that nominating O'Donnell would propel Coons to an easy victory in the largely Democratic state, saying "it's that simple." But those cautions were brushed aside, and O'Donnell exploited low voter turnout and rules barring independents from casting Republican ballots to seize 53.1 percent of the votes. Castle, whose career includes two gubernatorial victories and nine terms in Congress, captured 46.9 percent. It is his first loss.

"Republicans are in big trouble," said Jennifer Duffy, senior editor of The Cook Political Report. "They would not be favored to win this race anymore. She is a deeply flawed candidate."

O'Donnell's victory accelerates the pace of the tea party movement, adding to electoral successes in Utah, Kentucky, Nevada and Alaska. It also marks the first time that one of the eight Republicans to support the Democratic cap-and-trade bill in the House last year was cast out in a primary.

The race was a potent blend of recession economics, disdain for established politicians and sharp partisanship over the president's policies on health care and energy. O'Donnell sought to excite conservatives by repeatedly warning that Democrats would proceed with major climate legislation, including cap and trade, after their numbers are reduced in the November elections, but before losing lawmakers leave office in January.

'Nobody wants this [climate] bill'

She cautioned that Castle could be the deciding climate vote in the lame-duck session of Congress. She also described herself as casting the critical vote that would sink it. She portrayed the election as urgent because the winner in November will be appointed immediately to finish the final four years of Biden's term.

"Castle is a big supporter of cap and trade," O'Donnell said recently on FOX News. "Nobody wants this bill. In this race, I am the only candidate who has pledged to not only vote against it but advocate against it. So people are looking at this as perhaps me being that filibuster vote."

The race became the focus of conservatives in the past two weeks. Sarah Palin endorsed O'Donnell on Sept. 9, saying she is "against Obama's 'cap and tax' scheme" and the new health care law. Tea party leaders Sen. Jim DeMint (R-S.C.) and Rep. Michele Bachmann (R-Minn.) also weighed in on O'Donnell's behalf.

It sparked a late-campaign flurry. Palin recorded robocalls that peppered phone lines last weekend, and O'Donnell blasted out glossy mass mailings with side-by-side pictures of her and Palin, showcasing their physical and political similarities.

"STOP THE OBAMA-CASTLE-PELOSI AGENDA!" said a mailing delivered Monday, citing a popular warning this year by GOP challengers -- that incumbents are aligned with liberal House Speaker Nancy Pelosi (D-Calif.).

O'Donnell's conservative message resonated best in the state's southern countryside, where she needed a strong outcome to counter urban Republican moderates in the north. Sussex County is contained in a pair of geographic parentheses, with Maryland on the west and the Atlantic on the east. In between is a 50-mile stretch of tan corn rows drying in small fields abutting forest clumps and plots of soybeans.

The area, far to the south of Wilmington and the orbit of nearby Philadelphia, is sometimes called "lower, slower Delaware." It had electoral promise for O'Donnell: It was the state's only county to support Republican candidate John McCain during the presidential race in 2008. O'Donnell also tested well there that same year, when she nearly tied Joe Biden in votes when trying to unseat him from the Senate. More than 86,000 Sussex residents voted in that election, and Biden edged O'Donnell out with 272 ballots.